Printed Stretchable Graphene Conductors for Wearable Technology

Skin-compatible printed stretchable conductors that combine a low gauge factor with a high durability over many strain cycles are still a great challenge. Here, a graphene nanoplatelet-based colloidal ink utilizing a skin-compatible thermoplastic polyurethane (TPU) binder with adjustable rheology is developed. Stretchable conductors that remain conductive even under 100% strain and demonstrate high fatigue resistance to cyclic strains of 20–50% are realized via printing on TPU. The sheet resistances of these conductors after drying at 120 °C are as low as 34 Ω □–1 mil–1. Furthermore, photonic annealing at several energy levels is used to decrease the sheet resistance to <10 Ω □–1 mil–1, with stretchability and fatigue resistance being preserved and tunable. The high conductivity, stretchability, and cyclic stability of printed tracks having excellent feature definition in combination with scalable ink production and adjustable rheology bring the high-volume manufacturing of stretchable wearables into scope.


Graphite exfoliation into graphene nanoplatelets
As is described in the experimental section of the main text, raw graphite was intercalated with sulfuric acid and potassium permanganate and subsequently thermally expanded in a microwave before exfoliation into graphene nanoplatelets (GNPs). 1,2 This allowed high exfoliation at relatively low shear rates in order to maintain rather large platelets, resulting in highly conductive printed tracks. Figure S1 shows morphological changes induced to raw graphite by intercalation and thermal expansion. Figure S2 presents GNP size distributions obtained via dynamic light scattering (DLS) after removal of the largest platelets via 1h settling. The DLS data indicate a polydisperse GNP population with an average hydrodynamic diameter of 2.3 ± 0.4 µm. This polydispersity is corroborated by the micrographs in Figure S3. From the optical micrograph in Figure S3a, it becomes apparent that the sample contains some larger ~50 µm particles and many smaller flakes on the order of a micrometer (pixel resolution: 212 nm/pixel). From the thickness contrast, it can be concluded that we obtained multilayer graphene nanoplatelets that most likely will behave as stiff plates. 3 With scanning electron microscopy, the multilayer nature of the larger flakes could be resolved ( Figure S3b-d).

Rheology of GNP-based inks
In addition to the rotational rheology shown in Figure 1 of the main text, oscillatory rheology was performed to obtain a basic understanding of the viscoelastic behavior of the GNP-TPU inks. First, an amplitude sweep was performed with increasing shear strain γ from 0.01-100% and fixed angular frequency ω = 10 rad/s. From the results in Figure S4a and b, it might be concluded that the storage modulus G' is dominant over the loss modulus G" over almost the entire shear range, which is also apparent from the loss factor tan δ = G"/G' slightly below 1. The clear contributions from both G' and G'' to the ink rheology with a dominant loss modulus imply that the ink behaves like a viscoelastic solid. 4 We hypothesize that this behavior is due to the presence of a jammed platelet network. The loss factor gradually increases with shear strain until it rises more drastically at shear strains beyond 30%, which we attribute to destruction of the ink microstructure.
Next, a frequency sweep was performed by varying the angular frequency ω from 0.1 to 100 rad s -1 at fixed shear γ = 0.01% ( Figure S4c-d). At this small shear amplitude, both G' and G" and, hence, the loss factor are relatively frequency-independent. This is another indicator of a solid-like jammed network structure, which, at least at these low strain amplitudes, is able to elastically store energy in the form of network-like particle-particle interactions. 4,5 Figure S4. Oscillatory rheology of a typical GNP-TPU ink; a) storage (G') and loss (G") moduli and b) corresponding loss factor tan δ = G"/G' as measured in an amplitude sweep with γ varied from 0.01 to 100% and ω = 10 rad/s; c) storage (G') and loss (G") moduli and d) corresponding loss factor tan δ = G"/G' as measured in a frequency sweep with γ = 0.01% and ω = 0.1 -100 rad/s. All measurements were performed in duplicate and error bars indicate the standard deviation (some are invisible due to their small magnitude). , with R the resistance. A correction factor C of 0.52 was applied to correct for the coating geometry (55 x 12 mm) deviating from an infinite square. [1] ̅ s = 38 ± 6 Ω □ -1 mil -1 , corresponding to a conductivity S = (1.1 ± 0.2) · 10 3 S m -1 and a resistivity ρ = (9.6 ± 1.5) · 10 -4 Ω m; b) The baseline thickness was obtained according to a procedure detailed in section 3 of the SI based on a line of 12 mm (1.4 ·10 4 data points).

Conductor profiles and determination of baseline thickness
Profiles of printed conductors were rather rough ( Figure S7). In this work, the baseline thickness (orange continuous lines in Figure S7) was used as an indication of the active thickness of the conductive path, because it corresponds to the thickness of a homogeneous layer that contributes to conductivity. We expect that platelets sticking out do not contribute as much to in-plane conductivity. The baseline thickness was obtained according to the following procedure: The conductor profile was sampled with a Dektak Bruker XT profilometer with a resolution of 0.833 µm/pt, a stylus radius of 2 µm and a force of 3 mg. The manufacturer's software was used to level the dataset. For each print type, 3, 5 or 10 samples were profiled (Table S2), resulting in ≥ 9·10 3 data points per print type. For the data analysis, peaks and valleys were extracted from the dataset with the findPeaks and findValleys functions from the R package quantmod. 6 Negative datapoints were excluded from the dataset, as they would likely refer to pinholes or mistakes in the levelling process. Next, the mean peak height was calculated for all peaks > 10 µm. Next, the valley data points exceeding the mean peak height were discarded, as they likely refer to shoulder peaks. Finally, the mean baseline thickness was calculated from the remaining valleys. In Figure S7, the baseline, mean and peak thicknesses are plotted for representative samples of each conductor type. Table S2 provides the mean thickness and baseline thickness for printed conductors on each of the substrates. In both cases, the print layer is thicker on TPU ST604 than on TPU EU94 and PET ST504. This may be related to a difference in substrate roughness. The root- 7 with n the number of profilometer data points i, and yi the height at point i is 2.0 for PET ST504, 2.1 µm for TPU EU94 and 6.5 µm for ST604.  The (baseline) thicknesses were extracted from the profilometer datasets as described above.

b)
Resistances were measured with triplicate measurements on a Keithley 2612A System SourceMeter (4-point measurement). From these data, sheet resistances were obtained by dividing the line resistances by the number of squares between the electrodes and by using the baseline thickness to normalize to a layer thickness of 25 µm as described in the experimental section of the main text. c) The number/lines per sample refers to the number of profilometer tracks measured orthogonal to the conductor's longitudinal axis. This number of lines resulted in ≥ 9·10 3 data points/sample. 6. Abrasion resistance of screen-printed conductors Figure S8. Abrasion resistance of printed conductors formulated with two different GNP:TPU ratios printed on three different substrates. Tape tests were performed with Scotch® magic TM tape (3M), which was placed on the sample with a 1 euro coin (mass = 7.5 g, diameter = 23.25 mm, pressure = 173 N m 2 ) on top for 30 s. Tape samples on the right display the material removed from the printed tracks. The conductors formulated with a GNP:TPU ratio of 1:3 (as used in this work) are more abrasion resistant than the conductors containing GNP:TPU = 1:2.

Flexographic printing
A flexographic print test was performed using an ink similar to the one presented in the article. The ink contained 7.6 wt% graphene nanoplatelets (GNP) purchased from Sigma Aldrich (grade M5), 22.8 wt% thermoplastic polyurethane (TPU) binder, 1.1 wt% ethyl cellulose (EC) and 68.4 wt% solvent (dowanol PnB). Printing was performed on paper using an RK Flexiproof 100 flexo printer with a 1.7 mm thick printing plate designed for use with solvent-based inks. As shown in Figure S9, horizontal and vertical lines of various widths are printed fairly successfully on paper for all layer widths. Figure S9. Scan of a flexographic print on paper.  Figure S11. Optical micrographs of printed tracks on EU94 (left) and ST604 (right) before straining (ε = 0%), under uniaxial load (ε = 20, 50, 100%) and after final relaxation. Although the applied strain after final relaxation was 0%, the residual strain was 11.3% for EU94 and 34.0% for ST604. Table S3. Gauge factors (GF ε ) for selected strain levels ε during a strain test with increasing strain amplitude with ε increasing from 2 to 100% and a strain rate of 200 mm min -1 ( Figure  2). Sample GF2 GF4 GF10 GF20 GF30 GF40 GF50 GF60 GF70 GF80 GF90 GF100 EU94 13.1 11.6 12.

Serpentines vs straight lines: Electromechanical response
Serpentine-or meander-shaped printed tracks are often employed to mitigate the effects of straining on metal-based conductors. [8][9][10][11][12][13][14] The high fatigue-resistance and low gauge factors of our straight printed tracks make serpentine-shaped tracks obsolete for the GNP-based conductors studied in the current work. Nonetheless, serpentines could mitigate the rise in resistance of our printed conductors by a factor of almost two as is shown in Figure S12. For technological application, the tracks are sometimes cut before lamination. Cutting results in drastically reduced strain levels, because the cut samples are effectively just being unrolled rather than stretched during the strain test. This does not apply significant strain to the sample. It should be noted that laminated cut samples likely display a behavior intermediate between regular uncut serpentines and the cut versions shown here, tending more towards the unprocessed versions.

Residual strain of substrates due to rapid cycling
In this study, cyclic straining was performed rapidly (500 mm min -1 or 658% min -1 ) with insufficient time for complete substrate relaxation. Hysteresis and rate dependence of the mechanical behavior are a common phenomenon among thermoplastic polyurethanes. 15 As shown below in Figure S16 and Table S7, this resulted in residual strain (εres) levels of approximately 3% (EU94) and 8% (ST604) after exposure to cyclic strains of 20%. The substrate hysteresis was observed to increase during cycling, which means that the substrate remains extended. Thanks to this increasing residual strain, the effective strain εeff = 20% -εres during cycling reduces over time from 20% in cycle 1 to approximately 17% (EU94) or 12% (ST604) in cycle 1000 ( Figure S16c-d). The residual strain was extracted from the dataset for each cycle at the first point in the unloading cycle where the stress was equal to zero. The residual strain remained unaffected by photonic annealing.
This substrate hysteresis may be related to the observed behavior where an increase in residual strain correlates with a decrease in gauge factor ( Table 2) and resistance (Figure 3a).
In particular for ST604, there appears to be a correlation between the two effects.

Dynamic gauge factor
In the main text, the gauge factor was defined in equation 1: where Rmax,i represents the resistance at the maximum strain level of loading cycle i, R0 the initial resistance and ε the maximum tensile strain. For studying the dynamic behavior of the resistance during cycling, the dynamic gauge factor (DGF) would be more appropriate. 16 This may be defined as follows: Here, ∆Ri = Rmax,i -Rmin,i, with Rmin,i the resistance at the minimum strain level of loading cycle i, and εeff,i = 20% -εres,i. The resulting dynamic gauge factors are presented in Table S5.
During cycling, the dynamic gauge factors after the first cycle increase slightly over time but remain very low until they reach a close-to-stable level after cycle 500-800.
As hypothesized previously for stretchable composites of GNPs embedded in a PDMS matrix, 17 and for TPU-GNP composites with very low GNP loading, 18 these low DGFs may be due to rearrangement of GNPs inside the stretchable polyurethane matrix, thereby absorbing the strain while maintaining GNP connectivity. Interestingly, photonic annealing reduces the dynamic gauge factor as it evolves during cycling. We attribute this to a reduction of stretchability of the polyurethane matrix due to heating.
Finally, it should be noted that the dynamic gauge factor is strain rate-dependent. Slower cycling generally reduces the DGF and GF for TPU-containing conductive composites. 16,18,19 As strain rates in the current study were very high, the (D)GFs could likely be reduced by decreasing the cycling rate. Further studies featuring slower strain cycles would be necessary to understand the role of substrate hysteresis and the GNP-TPU network in the evolution of (dynamic) gauge factors and peak resistance during cycling.   Figure S17. Fit procedure to extract the loading and unloading elastic moduli from the stressstrain curves. The effective Young's moduli are determined from the first loading curve and the last unloading curve based on a least square fit with the solver in Microsoft Excel. It should be noted that the accuracy of the loading modulus is limited due to the limited number of datapoints in the fit procedure.

Gauge factors
12 Photonic annealing  Figure S18. Effect of photonic annealing with increasing energy level on the normalized resistance R/R0 in response to cyclic straining at 20% peak strain for conductors printed on EU94 (a-c) and ST604 (d-f). a,d) Development of the peak resistance R/R0 over 1000 cycles; b,c,e,f) R/R0 and strain during cycle 1-5 (b,e) and cycle 1000 (c,f). Rs values represent the averages derived from Table 1 multiplied by the R0/R0,p factor in Table 3. For EU94, these resistances were achieved with IPL energies E of 0.90, 1.4 and 2.3 J cm -2 , while the applied energies were 0.62, 0.95 and 1.6 J cm -2 for ST604. Pristine samples are presented in dark blue. The lines representing E = 0.62 and 1.6 J cm -2 in panel (e) do not start at 1, for the strain program got aborted during cycle 1, after which the measurement was restarted effectively in cycle 2; g) schematic representation of the photonic annealing process with a Xenon flash lamp emitting intense bursts of broad-spectrum light with wavelengths between 200 and 1500 nm. A mirror ensures that most light reaches the sample.
13 Wristband Figure S19. A wearable, stretchable and flexible wristband formed from a circuit composed of two printed and photonically annealed (E = 2.3 J cm -2 ) GNP tracks on EU94, a red LED (IF = 20 mA, UF = 2V) and a 3V lithium Energizer ® battery, which were connected with copper tape. Everything was laminated on a silicone bracelet using Neorez U-431 resin; a) wristband worn on the wrist; b) wristband under (limited) strain; c) circuit without silicone bracelet under significant strain.